The present invention generally relates to vehicle brake systems and more particularly relates to an electro-hydraulic brake system for motor vehicles.
An electro-hydraulic brake system of this general type is e.g. disclosed in the article xe2x80x98Das elektro-hydraulische Bremssystem von Continental Tevesxe2x80x94eine neue Herausforderung fxc3xcr die Systemxe2x80x94und Methodenentwicklung in der Seriexe2x80x99 (xe2x80x98The electro-hydraulic brake system of Continental Tevesxe2x80x94a new challenge for the series development of systems and methodsxe2x80x99), VDI Berichte No. 1547, 2000. On application of the brake pedal, the separating valves in the prior-art brake system are switched to assume their closed position on command of control signals generated by the electronic control and regulating unit. When the control or system electronics should fail, the travel simulator is blocked mechanically by one of the master cylinder pistons moving so that the travel simulator cannot receive pressure fluid volume in the event of actuation of the master brake cylinder in the emergency braking mode.
The time from the commencement of actuation of the master cylinder initiated by the driver until closing of the separating valves is problematic in the electro-hydraulic brake system of the state of the art. When the master cylinder actuation is too quick, the master cylinder piston blocking the travel simulator may move too far before the separating valves are closed. One consequence would be a xe2x80x98hardxe2x80x99 brake pedal what could irritate the driver, and another one that only a pressure sensor could still sense the extent of the deceleration demand.
An object of the present invention is to disclose an electro-hydraulic brake system of the type mentioned hereinabove permitting a very rapid change-over of the separating valves into the closed position.
This object is achieved according to the present invention by providing an additional electric circuit, which actuates the separating valve independently of the electronic control and regulating unit as the driver""s deceleration demand occurs. The reaction time of the electronic circuit is then considerably shorter than that of the electronic control and regulating unit. These measures achieve another advantage that is directed to the condition in which the electro-hydraulic brake system is not yet initialized (e.g. by switching on the ignition, door contact, remote control) but when the driver applies the brake pedal.
To render the idea of the present invention more specific, the electric circuit is arranged for to have an evaluating circuit for detecting the driver""s deceleration demand that serves to enable the actuation plus the actuation of the separating valve.
According to a favorable aspect of the subject matter of the present invention, a timer is connected downstream of the evaluating circuit to generate an actuating signal for the separating valve and suppress the actuation of the separating valve upon expiry of a predetermined duration. It is achieved by this measure that the separating valve is disconnected after a preset time, thereby avoiding a safety-critical performance in case the system does not function properly.
A second safety function may be implemented in that a hardware monitoring circuit (watchdog) is connected downstream of the electronic control and regulating unit, said monitoring circuit suppressing the actuation of the separating valve in the event of a dynamic status signal produced by the electronic control and regulating unit differing from a status signal produced by the hardware monitoring circuit (watchdog).
In this arrangement, the above-mentioned actuation of the separating valve is effected by means of two actuation logic circuits being associated with a safety switch and a separating valve driver connected in series with the separating valve.